The present invention relates to a light exposure device and a method for exposing a pattern, specifically a semiconductor IC pattern formed by photographing technique such as a mask or reticle to a substrate such as a silicon wafer, bubble wafer, ceramic substrate or a printed circuit board.
In forming an LSI pattern on a wafer, it is conventional to apply photoresist on the LSI wafer and expose a photomask pattern on the wafer to expose the photoresist thereon.
This process is called lithography and it includes contact method (in which exposure is carried out with the mask and the wafer being in contact), a proximity method (in which exposure is carried out with the mask and the wafer being spaced from each other by several microns to several tens microns) and a projection method (in which a pattern on the mask is projected onto the wafer). An X-ray exposure device has also been known.
FIG. 1 shows a construction of a projection type light exposure device as an example of the prior art light exposure devices. A pattern 2 indicated by an arrow on a surface of a mask 1 is focused to a wafer 4 by an exposure light source 3 through a minor optical system comprising a concave mirror M.sub.1, a convex mirror M.sub.2 and a plane mirror M.sub.3. Numeral 5 denotes an imaged pattern. Periphery of the mask 1 is drawn by suction to a carriage 6 by vacuum suction means with a flatness of .+-.2 microns. The carriage 6 which carries the wafer 4 and the mask 1 scans reciprocally as shown by an arrow A so that the mask pattern 2 is transferred onto the wafer 4 as the carriage 6 scans. A wafer chuck 7 forms an air path to link bores formed on the top thereof to a bore 8 formed at the bottom so that the wafer 4 is drawn to the surface thereof. The bore 8 is connected to a vacuum source, not schematically represented, through a pipe shown by an arrow 9. FIGS. 2a and 2b show an enlarged sectional view and a plan view, respectively, of the wafer 4 mounted on the wafer chuck 7. In FIG. 2b, the flatness of the wafer 4 is shown by contour lines 10. In the illustrated example, the wafer 4 is spherical and protrudes upward. When the surface of the wafer 4 does not coincide with an imaging plane 12 of the optical system as shown in FIG. 2a, only a portion of a line pattern 13 (FIG. 2b) is transferred because it is imaged only to that portion of a wafer surface 11 which lies within a depth of focus 14. The depth of focus 14 on the imaging plane 12 is determined by a resolution power of the optical system necessary to the transfer, and the depth of focus at the resolution power necessary to transfer a line width of 3 .mu.m is .+-.5 .mu.m. When this depth of focus is combined with a mask positioning error of .+-.2 .mu.m, an allowable depth of focus on the part of wafer is .+-.3 .mu.m. When the line width to be transferred is 2 .mu.m, the allowable depth of focus on the part of wafer is .+-.2 .mu.m. On the other hand, the flatness of the wafer surface 11 sucked on the flat chuck surface 15 is normally no less than .+-.6 .mu.m, sometimes more than .+-.10 .mu.m. This leads to the reduction of the yield. On the other hand, the improvement of the flatness of the wafer 4 per se is very difficult to attain.
It has been proposed to focus to the wafer surface 11 in order to image and print the mask pattern on a wafer of low flatness. Referring to FIG. 3, printing of a circuit pattern on the wafer by the focusing method is explained. The wafer 4 is spherical and protrudes upward as in FIGS. 2a and 2b and it is assumed that a depth of focus covers two contour lines 10. When a scan is made in the direction of arrow A (FIG. 1) with a printing width W.sub.1 without focusing, the circuit pattern is printed only to a ring-shaped area having a width R.sub.0 corresponding to the depth of focus. Then, as the scan is made with the same width W.sub.1 while focusing is made along the center of the width W.sub.1, the pattern is printed to the range of R.sub.1. Then, as the scan is made with a narrow printing width W.sub.2 three times one for each of three vertically divided areas with focusing, the pattern is printed to the range of R.sub.3. It is seen from the above explanation that divided exposure is best for a high yield. However, when three-division method is used, a yield or output is reduced by the factor of three resulting in a considerable amount of loss, and a joining of the individual printed patterns is very difficult to attain. Accordingly, a high precision pattern cannot be attained. Furthermore, complex mechanism and operation are required to detect focusing points and adjust focus planes. Accordingly, a cost is high and maintenance is difficult and troublesome.
While the example of LSI has been explained above, the same is equally applicable to other substrates (such as magnetic bubble substrate, thin or thick film substrate and printed circuit board).